The present invention relates to a process for the crystallization of dicarboxylic acids.
More specifically, the present invention relates to a novel process for the crystallization of dicarboxylic acids employing the use of additives which help increase the yield and purity of crystals.
The object of the invention is to achieve the desired increased size, high purity and greater yield of crystals.
The other object of the invention is to achieve a crystallization process in which crystallization is at a faster rate as compared to the conventionally known processes.
Still another object of the invention is to achieve the desired results by consuming less energy.
The embodiment of the invention resides in the crystallization of di-carboxylic acids in a novel crystallizer comprising a cylindrical impeller and impeller shaft.
Surfactants and/or salts are used as additives to achieve the desired increased size of crystals and high yield.
Addition of surfactants as additives for the crystallization of dicarboxylic acids results in obtaining bigger crystals with high purity, whereas addition of salts as additives for the crystallization of dicarboxylic acids results in the greater yield of crystals.
The addition of surfactants and salts result in obtaining high yield crystals of having bigger crystal size.
The modification of the crystal morphology and structure is achieved by introduction of additives which stick on one of the preselected crystal faces thereby inhibiting the growth of the crystal predictably perpendicular to the surface on which they adhere. The size of the crystal structure is drastically varied by carrying out crystallization in electrolyte solutions.
U.S. Pat. No. 5,296,639 claims a process for purification of adipic acid during crystallization by modifying crystal morphology through the introduction of additives which are among caproic acid and surfactants, thereby decreasing the incorporation of impurity on the crystal system.
U.S. Pat. No. 5,827,700 teaches a process of recovery and crystallization of citric acid from impure process stream by introducing very strong salts.
Juetten, in his thesis entitled xe2x80x9cThe Enhanced Crystallization of Dicarboxylic Acids in Electrolyte Solutionsxe2x80x9d, Michigan State University, 1992, performed crystallization experiments on aqueous solutions using dicarboxylic acids in order to determine by trial-and-error which electrolytes caused xe2x80x9csalting outxe2x80x9d.
One of the major drawbacks associated with the use of salts, either buffer salts or acid salts is the instant precipitation, which results in the smaller size of the crystals. In the subject invention to overcome this drawback, the selection of salts with the surfactants has resulted in achieving the desired crystal size along with better yield of crystals.
The present invention relates to a process for crystallization of dicarboxylic acids in a system, which is under continuous shear force that is created in the annular space between the impeller and the crystallizer wall. The system consists of selection and addition of additives in a manner to avoid any effect on the crystal purity.
To achieve the objectives of increased size, high purity and greater yield of crystals, in the present invention crystallization of diacids is carried out in a solution consisting of a solvent or a mixture of solvents, crude diacid and the additives in a crystallizer.
The crude diacids consists of one solute in major amount and impurities in minor amounts. When re-crystallized, the subject process provides a highly purified crystals of the diacids, as the amounts of impurities present, if any, are totally dissolved in the solvent/solution.
The solvents used in the present invention are selected preferably from water, any aliphatic monobasic acid having 2-4 carbons comprising primary and secondary hydrogen, any primary alcohol having 2-4 carbon or any combination thereof. Preferably, the solvent used in the subject process is water or water with small amount of acetic acid, wherein the ratio of water to acetic acid is more than 1:1. More preferably, the solvent with less than 10% of acetic acid is used. The preferred range of the solvent to the crude diacid is between 10 to 1 to 0.1 to 1. Most preferred range of the solvent to the crude diacid is 5 to 0.2
The additives used in the subject invention are selected from surfactants, and/or buffer salts and acid salts.
The surfactants addition results in obtaining bigger crystals with high purity. The surfactants are selected from non-ionic surfactants, anionic surfactants, cationic surfactants and zwifterionic surfactants. Preferably, the anionic surfactants are used. The preferred anionic surfactants can be selected from Sodium dodecyl sulfate (SDS), Sodium dodecyl benzene sulfonate (SDBS) and Sodium bis (2-ethyl hexyl) sulfosuccinate (AOT).
The nonionic surfactant used in the present invention are selected from hexoxy ethylene glycol mono-n-dodecyl ether (C12E6) and Tween 20.
The cationic surfactants can be selected from cetyl trimethyl ammonium bromide (CTAB) and dodecyl dimethyl ammonium bromide (DDAB).
The zwitterionic surfactants are selected from Phosphatidyl choline (PC) and Phosphatidyl ethanolamine (PE).
The amount of surfactants that is required for achieving good crystal size and shape is between 0.005% to 2.0% of the weight of the total solution. It has been observed that the concentration of surfactant below the critical micellar concentration (CMC) in the solution has achieved the desired results successfully. Most preferably, the amount of surfactant used is between 0.01% to 1.0% i.e. below the CMC.
To increase the yield of crystals the buffer acids and/or buffer salts are used as additives. The addition of salts results in increased rate of crystallization thus resulting in greater yield of crystals. The buffer salts exhibit resistance to the change in pH and acid salts increases the pH. The buffer salts used in the present process are selected from Na2HPO4, NaH2PO4, KH2PO4 and K2HPO4. Preferably these salts are selected from Na2HPO4 or K2HPO4 or a combination thereof. The amount of buffer salts taken is between 0.0025% to 1.0% of the weight of the total solution.
The acid salts used in the present invention are selected from NaHSO4 and KHSO4, more preferably KHSO4. The amount of acid salts taken is between 0.0025% to 1.0% of the weight of the total solution.
Hence, in the subject crystallization process, both the acid and buffer salts along with the surfactants are used to achieve the desired average crystal size as surfactants makes the crystal grow and the salts precipitate out of the di-carboxylic acid.
The presence of surfactants results in growth of crystals, thereby results in obtaining crystals of higher purity. Salts present in the system results in the increased yield i.e. the weight of the crystals formed is more than the weight of crystals formed in a conventional process. The same is shown in the comparative table:
The amount of surfactants and buffer or acid salts is between 0.005% to 4.0% of the weight of the total solution.
As can be clearly seen from the Table-1, the presence of surfactants helps only in the growth of the crystals and the size of the crystal is increased by three times approximately. Increased size results in increasing the purity of crystals. Here, the yield increases but the increase is not considerable.
In case of the use of salts only as additives, the size of the crystals decreases, but the yield increases.
However, in the presence of both the salts and surfactants, there is increase in the yield, purity and size of the crystals.
In accordance with the present invention the diacid is crystallized in medium consisting of a suitable solvent, which is more preferably a mixture of water and acetic acid in different proportions and additives which are selected from surfactants, buffer salts or/and acid salts in a novel crystallizer having a impeller with impeller shaft.
In the process for the crystallization of di-carboxylic acid, the crude diacid at an acid value of 15-35, is put in the annular space, between the impeller and crystallizer wall. The crude diacid is dissolved in solvent at a suitable temperature at which the crude diacid is dissolved in the solvent to form a solution. The said solution is evaporated till a final acid value of 18-64 is achieved at a temperature of between 60-75xc2x0 C. The resultant solution is then treated with additives and is cooled to 25-35xc2x0 C. and crystallized for one hour, which results in the crystals of the di-acid to purge out of the solution which are later recovered by filtration/centrifugation.
Accordingly, the subject invention relates to a process for the crystallization of di-carboxylic acids comprising: dissolving the crude dicarboxylic acid at a temperature between 30xc2x0 C.-75xc2x0 C. to dissolve the crude diacid in the solvent in an evaporator whereby portion of the said solvent evaporates increasing the concentration of the dicarboxylic acid in the said solution, transferring the said solution from the evaporator to a crystallizer, adding the additives selected from surfactant, buffer salts and/or acid salts or mixture thereof in the said evaporated solution in the said crystallizer, cooling the said mixture resulting in the formation of crystals of dicarboxylic acids, wherein the said crystallization takes place in the annular space between the impeller and the crystallizer wall at 10-80xc2x0 C.
The present invention relates to a process for the crystallization of dicarboxylic acid comprising dissolving the crude dicarboxylic acid at a temperature between 30xc2x0 C.-75xc2x0 C. to dissolve the crude diacid in the solvent in an evaporator whereby portion of the said solvent evaporates increasing the concentration of the dicarboxylic acid in the said solution, transferring the said solution from the evaporator to a crystallizer, adding the additives selected from surfactant, buffer salts and/or acid salts or mixture thereof in the said evaporated solution in the said crystallizer, cooling the said mixture resulting in the formation of crystals of dicarboxylic acids, wherein the said crystallizer comprises solid cylindrical impeller conforming to the shape of the crystallizer rotated by means of cylindrical shaft attached to the speed motor, the said cylinder coaxial impeller or series of such impellers having different diameters separated from each other, and each of these cylinders moving at different speeds with the innermost impeller having the diameter between 3-5 cm, moving at highest speed preferably at the rate of 10-50 RPM and the outermost impeller having the diameter of between 5-10 cm, moving at lowest speed preferably at the rate of 1-10 RPM, rotated by a common shaft which is attached to the motor, wherein crystallization takes place in the annular space between the impeller and the crystallizer wall at 10-80 degrees C. maintaining the annular space between the two cylindrical impellers between 1 mm-50 cms.
The crystal forms are needle shaped, monoclinic crystals. The additives modifies the shape of crystal by increasing the leniency and decreasing the surface area.
Dissolution of the crude dicarboxylic acid in a solvent and the addition of additives in the said solvent can be carried in a separate evaporator out side the said crystallizer.